The self-sticking adhesive on the bottom of the stencil seals around each BGA pad to ensure that solder paste will not bleed under the stencil when the paste is applied.
Disposable CircuitMedic Stencils are easy to use and will not leave any residue on the board surface.

If you currently use metal stencils you need to fixture them or tape them in position to hold them in place.
Metal stencils can warp, and if the circuit board has undulations in the board surface, the metal stencil will not sit flat.

Since there is no gasket-like seal, solder paste can easily bleed under metal stencils when paste is applied with a squeegee.
Also solder paste can spill out over the sides of flat stencils contaminating the circuit board surface.